ADS in the game as it stands is a tricky business. I think you can script every weapon to have sniper zoom, but then everything would use the sniper scope sprite as there's only one. It can be changed, but it won't fit every weapon.The other alternative is changing the idle animation so you aim, but that means every time you stop, you stare down the sights. There are a few mods of this type hanging around, I have most of them.

ADS as an actual game effect would be nice for those of us who like that sort of thing, but it's ultimately useless as a game mechanic. The game's too fast to properly allow it and the weapons are already balanced for hip fire.

The game would maybe be a bit slower in the normal campaigns but imagine custom maps created just for that, with some special infected with vulnerable spots really hard to aim, very far ranged zombies or stuff like that, it would completely change the tone of the game and it could be quite interesting as an alternative to fast-paced campaigns.